Steerable wheel assemblies for vehicles pivot around an inclined axis known as the steering axis, the position of which is determined by the joints and suspension components of the wheel and steering assembly. Proper positioning and alignment of a steerable wheel, and especially a front wheel of a vehicle, may require the setting of toe, camber angle and caster angle. Toe is the angle between the vehicle's longitudinal axis and a plane through the center of the steered wheel/tire and affects the straight ahead running of the vehicle as well as steering. Camber angle is the inclination of the wheel axis toward the road surface in a vertical plane and is negative when the top of the wheel is inclined toward the center of the vehicle. Camber angle influences lateral control and is typically moderately negative. Caster is the distance between the contact point of the wheel/tire with the road and the point at which the steering axis intersects the road as viewed from the side of the wheel. Caster determines the degree of self-centering action in the steering as well as influencing straight-line stability and steering force in curves and is typically positive, i.e., the top of the steering axis is tilted rearwardly. Caster angle is defined as the angle between the steering axis and a vertical plane, perpendicular to the longitudinal centerline of the vehicle, as viewed from the side of the vehicle. During assembly and/or repair of vehicles, it is important to measure, adjust or audit, and set the toe, as well as camber and caster angles of a steerable wheel so that the vehicle will drive and steer properly. In order to set or audit the caster, it is necessary to measure the caster angle.
In the past, caster angle was measured using various direct and indirect methods which were time consuming, labor intensive, and less accurate than desired. Direct caster angle measurement methods require human operators or mechanisms to place measurement tools in contact with the vehicle and are subject to placement error and wear. Indirect caster angle measurement methods require an operator or mechanism to turn the steering wheel of the vehicle to different positions while having the toe and camber measurements recorded after which known formulas are used to calculate the caster. This indirect method allows measurement error in each of the steering wheel positions as well as operator error and/or mechanical wear. If the toe and camber of the wheel are measured by contact methods, then additional error can be encountered. Moreover, the indirect measurement method requiring measurements at various steering wheel positions requires significant cycle time to perform thereby causing greater expense.
Therefore, a need was determined for an apparatus and method for measuring the caster angle of a wheel assembly on a vehicle during vehicle assembly or repair which would allow rapid, accurate determination of the caster angle without contact with the wheel assembly thereby allowing the caster of the wheel assembly to be properly adjusted or audited and set before the vehicle is released for sale or re-use.